1. Field of the Invention
The invention relates to a composite multilayer material for plain bearings, comprising a ternary layer, a bearing metal layer, a diffusion barrier layer and an overlay applied by electroplating. The invention also relates to a process for the production of bearing shells.
2. Description of Related Art
Composite multilayer materials with a structure comprising steel backing/lead-bronze/lead-tin-copper overlay have proven very reliable and to have a high mechanical load carrying capacity. Such composite multilayer materials are described in Glyco Ingenieurberichte 1/91, for example.
A typical representative of these groups of materials has the following structure:
steel PA1 CuPb22Sn bearing metal PA1 nickel barrier PA1 PbSn10Cu2 overlay. PA1 embeddability with respect to foreign particles PA1 running-in or conformability of sliding counterparts PA1 corrosion protection for the lead-bronze PA1 emergency running properties in the case of an oil shortage.
The electrodeposited overlay in particular is a multifunctional material, which, inter alia, takes on the following tasks:
The bearing metal likewise contains certain emergency running reserves in case the overlay is completely worn away.
These bearing designs, which have proven effective for decades, today still contain quantities of lead in the bearing metal and the overlay. This is unsatisfactory from the point of view of the environmental impact of heavy metals.
The only lead-free plain bearings currently produced are aluminium-based. Two-component bearings comprising an aluminium alloy on steel are preferably used. Alloy elements in the aluminium are preferably tin, silicon, nickel and copper. Disadvantages of these groups of materials are their low fatigue strength, low notch sensitivity, low heat resistance, insufficient embeddability and relatively poor heat conductivity.
Electrodeposition has hitherto mainly been carried out using baths containing fluoroborate. Copper deposition could be carried out only up to a rate of 2 vol. % in these baths, while in cyanide baths deposition rates for the copper of up to 20 vol. % could be achieved. However, it has become clear that the coating is extremely brittle and in this respect is not very durable.
DE-OS 27 22 144 discloses the use, as a soft metal bearing alloy for multilayer plain bearings, of an alloy comprising more than 6 to 10 wt. % copper and 10 to 20 wt. % tin, the rest being lead. This alloy may, inter alia, be applied by electrodeposition, wherein a nickel intermediate layer is provided as a diffusion barrier. However, this known alloy, which is produced using conventional electrolyte baths, exhibits coarse tin distribution.
The coating of electronic components is described in "galvanisches Abscheiden von Zinn-Blei aus Fluoroborat und fluorboratfreien Elektrolyten" ("Electrodeposition of tin-lead from fluoroborate and fluoroborate-free electrolytes") by H. van der Heijden in "Metalloberflache" ("Metal surface") 39 (1985) 9, pages 317-320. It has been shown that tin and lead may be easily dissolved in various organic sulphonic acids with the general formula RSO.sub.3 H, wherein R represents an alkyl group. The sulphonic acids are described as completely stable during electrolysis. The use of such fluoroborate-free baths for electroplating of sliding elements is mentioned, as are suitable additives necessary for the production of sliding elements.
Aqueous, acidic solutions are known from DE 39 02 042 A1 for the electrolytic deposition of tin and/or lead-tin alloys. These binary electroplating baths are used as corrosion protection, e.g. for printed circuit boards and batteries, wherein a relatively soft layer is deposited which is not wear-resistant and thus cannot be used for sliding elements. In addition to metal salts, inhibitors and free alkanesulphonic acids, these known electroplating baths comprise brighteners, which make it impossible to use these baths for example as ternary baths. It has been shown that, when copper salts are added for example, only a maximum of 1% copper may be deposited, because the bath composition is essentially suited to tin. JP 02/93096A (Pat. Abstract of Japan, C-732, Vol. 14/No. 294) describes a process for producing sliding elements, in which the overlay of Pb--Sn--Cu is applied to the prefabricated semi-finished product by electroplating. To this end, a fluoroborate-free electrolyte is used which does not contain brighteners but does contain free alkylsulphonic acid and non-ionic wetting agents. With these known electroplating baths, however, again only coarse tin deposition is achieved, wherein the layer structure is generally irregular. The high demands made of plain bearings are not met wholly satisfactorily.
DE 195 45 427 likewise describes fluoroborate-free electroplating baths for the production of overlays of lead-tin-copper, said baths containing a grain-refining agent comprising carboxylic acid and a fatty acid glycol ester. In this way, finely crystalline deposition of the tin is achieved with completely homogeneous distribution thereof, copper deposition of from 2 to 16 wt. % being obtained.
The prior German Application DE 196 22 166 describes ternary layers with hard particles, wherein fluoroborate-free electroplating baths with added alkylsylphonic acid are used for the overlay, to obtain a completely homogeneous distribution of the hard particles in the matrix material. In addition to lead-containing overlays, these alkylsulphonic acid baths may also be used for the application of lead-free layers of SnCuNi, Sn, SnCu and CuSn. However, it has been shown that, although copper contents of up to 16% may be obtained with these lead-free layers, these layers do not exhibit satisfactory properties as far as mechanical load carrying capacity and fatigue strength are concerned.